PSI - Issue 57

Pierrick Lepitre et al. / Procedia Structural Integrity 57 (2024) 395–403 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Experimental setup and methods

2.1. Test setup

Self-heating tests were performed on a ZwickRoell Amsler Vibrophore 50 (ZwickRoell GmbH & Co. KG, Ulm, Germany) at various loading frequencies and load ratios. Polished cylindrical unnotched test coupons (K t = 1) were used with two geometries:

• 50 mm² cross-section and 16 mm gauge length fatigue coupon, noted F50, • 25 mm² cross section and 15 mm gauge length fatigue coupon, noted F25.

A self-heating under cyclic loadings test is composed of a series of short loading blocks of increasing amplitude. Each self-heating test is characterized by its equivalent time τ eq , a term representing the coupon thermal loss. Each loading block is characterized by its loading amplitude σ a and mean stress σ x and can be decomposed in 5 parts (represented in Figure 1):

• A first step to reach the mean stress ( σ x ) of the block, • A temperature stabilization period of at least 5 τ eq , • A second temperature stabilization period of 30 seconds, • The solicitation of at least 5f r τ eq number of cycles at the block loading amplitude σ a , • A cool down time of at least 5 τ eq at σ x .

Prior doing a self-heating test, an estimation of τ eq is required. On a loading block of a self-heating test, for a relatively low loading amplitude (below the fatigue limit) but with enough thermalsignal, τ eq can be estimated with the post-processing protocol. Tests are instrumented with three K-type thermocouples with a recording frequency of 1 Hz, to measure the test coupon temperature, with thermocouple placed in the middle of the useful gauge, and both top and bottom machine jaw temperatures. Temperatures are recorded during the third, fourth and fifth phases (Figure 1) of the protocol. To prevent environmental influence on thermal loss, the Amsler Vibrophore is protected inside a box.

Figure 1 – Theoretical temperature rise and stress evolution during a self-heating block

2.2. Post-processing protocols By considering the specific Helmholtz free energy, for small temperature variation and small strains, the 0D heat